It is known that the winemaking process involves two phases in the same vat: a must phase consisting of grape bunches or berries, and a juice phase, in which the fermentation phenomena occur, wherein an effort is made to control and to optimize the quantitative and especially qualitative yield of the resulting wine.
However, this optimization poses numerous problems because of the considerable differences existing between the preferred temperatures for each of these phases.
During the fermentation process of the must phase, the temperature can rise to 40.degree.-45.degree. C., and it would be advisable to maintain it at about 30.degree.-35.degree. C. to promote the finishing of the maturation and intracellular maceration and to extract the color better without risking stopping this fermentation because of too high a temperature. On the other hand, in the juice phase, a temperature on the order of 18.degree. C. is the most favorable to assure the transformation of the sugar into alcohol and the development of aromas.
Further, it should be noted that the vintage temperature itself can vary greatly depending on whether it was performed in hot weather (risk of overheating of the bunches) or in cold weather (too slow starting of the fermentation) which at the start requires heating of the bunch-juice batch.
Various devices are now on the market for cooling and heating of musts and juices.
A device described in French Patent A-827 482 consists of a rigid coil placed in a stationary position on the inside of the vat about 0.15 meters above the marc layer, which limits its use to destemmed and treaded vintage.
This coil is integral with the vat and therefore cannot follow the progression of the marc cap, particularly in the case of winemaking of entire bunches.
Some devices perform the cooling or heating by pumping the juice in the lower part of the vat and making it pass into a stainless steel exchanger to send it back over the marc cap forming in the upper part of the vat.
However, these devices have certain drawbacks. Actually, putting the juice in motion by means of pumps, whose oenological qualities are often mediocre, often degrades the quality of the wine.
On the other hand, the phenomena of overheating or overcooling linked to anarchical deliveries or to clogging, which are often poorly controlled, are sources of risks.
In the case of stainless steel vats, cooling is often effected by running water around the vat.
However, it should be noted that, because of the large diameter of these vats, this process cannot assure a cooling of the core of the products that they contain.
Moreover, the strong aerations necessary to promote evaporation of the water are often not observed.
This process makes possible more of a maintenance of temperature than a real cooling; further, it acts on the entire batch of material contained in the vat, without it being possible to act selectively on the must phase or the juice phase.
Stainless steel vats surrounded by hot or ice water circulation devices are also used. The drawbacks of these devices are numerous: phenomena of overheating or overcooling on the exchange surface are noted; the large diameter of the vats do not make it possible to guarantee a real effectiveness in the vat unit.
Further, the temperature gradient promotes too much convection phenomena which are not desirable if it is is desired to optimize the winemaking by means of stratification of temperatures.
Finally, according to some processes, exchangers generally called "flags," are immersed in the vat. These exchangers are immersed vertically in the vats and receive a circulation of heat-exchanging fluid which creates a convection movement.
The efficiency of these systems is very debatable because of the slightly thermal conductivity of the must phase, which is not very rich in juice.
On the other hand, handling of these expensive "flags" is difficult because of their weight, their bulk and their poor impact resistance.
Finally, and as in the processes mentioned above, they promote the convection phenomena to the detriment of the phenomena of stratification of temperatures.